In 1914 the Big Ten 'recommended' adding jersey numbers. Gophers coach Doc Williams didn't want his players to be identifiable to the other team, so he ignored the recommendation. By 1920 Minnesota was the only team in the Big Ten without uniform numbers, and the conference made them update their jerseys.
Old Doc Williams was on the hot seat, 1921 would be his final season. The great Minnesota coach had led some of the best teams Minnesota ever had, but the team was on the downturn. Clearly Doc Williams thought there was some advantage to having his players compete without uni numbers, and he didn't want to give that up easily.
Coach "Doc" Henry L Williams on the left, in 1921. via MNHS
For the second game of the 1921 season Williams had his players in jerseys with four digits Coach Williams told Northwestern Coach McDevitt before the game about his plan and McDevitt was fine with the stunt. However there were enough complaints to make the four digit uniforms a one time occurrence.
From the Minnesota Alumni Weekly:
Minnesota has adopted numbers of so many digits as to make them practically illegible. Bad. Minnesota can afford to live up to the spirit as well as the letter of the rule.
The Gophers next weeks opponents Ohio St. complained so much that Coach Williams was forced to apologize to the entire Conference. He half apologized, saying that the whole thing was a "little joke on the rulesmakers". It doesn't seem like Ohio St. needed Minnesota to wear jersey numbers as they beat Minnesota 27-0.
It's not a great picture but from the October 9, 1929 Minneapolis Tribune you can clearly see Minnesota QB Gilstad wearing number 2335.
The team then went with the same black jerseys with white number cut ins for the rest of the season. Lame.